<chapter id= "what-is-harfbuzz" >
<title > What is HarfBuzz?</title>
<para >
HarfBuzz is a <emphasis > text-shaping engine</emphasis> . If you
give HarfBuzz a font and a string containing a sequence of Unicode
codepoints, HarfBuzz selects and positions the corresponding
glyphs from the font, applying all of the necessary layout rules
and font features. HarfBuzz then returns the string to you in the
form that is correctly arranged for the language and writing
system.
</para>
<para >
HarfBuzz can properly shape all of the world's major writing
systems. It runs on virtually all operating systems and software
platforms, and it supports all of the standard font formats in use
today.
</para>
<section id= "what-is-text-shaping" >
<title > What is text shaping?</title>
<para >
Text shaping is the process of translating a string of character
codes (such as Unicode codepoints) into a properly arranged
sequence of glyphs that can be rendered onto a screen or into
final output form for inclusion in a document.
</para>
<para >
The shaping process is dependent on the input string, the active
font, the script (or writing system) that the string is in, and
the language that the string is in.
</para>
<para >
Modern software systems generally only deal with strings in the
Unicode encoding scheme (although legacy systems and documents may
involve other encodings).
</para>
<para >
There are several font formats that a program might
encounter, each of which has a set of standard text-shaping
rules.
</para>
<para > The dominant format is <ulink
url="http://www.microsoft.com/typography/otspec/">OpenType</ulink> . The
OpenType specification defines a series of <ulink url= "https://github.com/n8willis/opentype-shaping-documents" > shaping models</ulink> for
various scripts (including Indic, Arabic, Hangul, Hebrew, Khmer,
Myanmar, Thai and Lao, Tibetan, and a Universal Shaping Engine
designed to cover other scripts). These shaping models depend on
the font including certain features in its <literal > GSUB</literal>
and <literal > GPOS</literal> tables.
</para>
<para >
Alternatively, OpenType fonts can include shaping features for
the <ulink url= "https://graphite.sil.org/" > Graphite</ulink> shaping model.
</para>
<para >
TrueType fonts can also include OpenType shaping
features. Alternatively, TrueType fonts can also include <ulink url= "https://developer.apple.com/fonts/TrueType-Reference-Manual/RM09/AppendixF.html" > Apple
Advanced Typography</ulink> (AAT) tables to implement shaping
support. AAT fonts are generally only found on macOS systems.
</para>
<para >
Text strings will usually be tagged with a script and language
tag that provide the context for text shaping. <ulink
url="https://docs.microsoft.com/en-us/typography/opentype/spec/scripttags">Script</ulink>
and <ulink
url="https://docs.microsoft.com/en-us/typography/opentype/spec/languagetags">language</ulink>
tags are defined by OpenType.
</para>
</section>
<section id= "why-do-i-need-a-shaping-engine" >
<title > Why do I need a shaping engine?</title>
<para >
Text shaping is an integral part of preparing text for
display. Before a Unicode sequence can be rendered, the
codepoints in the sequence must be mapped to the glyphs
provided in the font, and the glyphs must be positioned
correctly relative to each other. For many of the scripts
supported in Unicode, these steps involve script-specific layout
rules.
</para>
<para >
Text shaping is a fairly low-level operation. HarfBuzz is
used directly by graphic rendering libraries such as Pango, as
well as by the layout engines in Firefox, LibreOffice, and
Chromium. Unless you are <emphasis > writing</emphasis> one of
these layout engines yourself, you will probably not need to use
HarfBuzz: normally, lower-level libraries will turn text into
glyphs for you.
</para>
<para >
However, if you <emphasis > are</emphasis> writing a layout engine
or graphics library yourself, you will need to perform text
shaping, and this is where HarfBuzz can help you.
</para>
<para >
Here are some specific scenarios where a text-shaping engine
like HarfBuzz helps you:
</para>
<itemizedlist >
<listitem >
<para >
OpenType fonts contain a set of glyphs (that is, shapes
to represent the letters, numbers, punctuation marks, and
all other symbols), which are indexed by a <literal > glyph ID</literal> .
</para>
<para >
The glyph ID within the font does not necessarily correlate
to a predictable Unicode codepoint. For instance, some fonts
have the letter " a" as glyph ID 1, but many others do
not. To pull the right glyph out of the font in order to
display " a" , you need to consult the table inside
the font (the <literal > cmap</literal> table) that maps Unicode
codepoints to glyph IDs. In other words, <emphasis > text shaping turns
codepoints into glyph IDs</emphasis> .
</para>
</listitem>
<listitem >
<para >
Many OpenType fonts contain ligatures: combinations of
characters that are rendered as a single unit. For instance,
it is common for the <literal > fi</literal> letter
combination to appear in print as the single ligature glyph
" fi" .
</para>
<para >
Whether you should render an " f, i" sequence
as <literal > fi</literal> or as " fi" does not
depend on the input text. Rather, it depends on the whether
or not the font includes an " fi" glyph and on the
level of ligature application you wish to perform. The font
and the amount of ligature application used are under your
control. In other words, <emphasis > text shaping involves
querying the font's ligature tables and determining what
substitutions should be made</emphasis> .
</para>
</listitem>
<listitem >
<para >
While ligatures like " fi" are optional typographic
refinements, some languages <emphasis > require</emphasis> certain
substitutions to be made in order to display text correctly.
</para>
<para >
For example, in Tamil, when the letter " TTA" (ட)
letter is followed by " U" (உ), the pair
must be replaced by the single glyph " டு " . The
sequence of Unicode characters " டஉ" needs to be
substituted with a single " டு " glyph from the
font.
</para>
<para >
But " டு " does not have a Unicode codepoint. To
find this glyph, you need to consult the table inside
the font (the <literal > GSUB</literal> table) that contains
substitution information. In other words, <emphasis > text shaping
chooses the correct glyph for a sequence of characters
provided</emphasis> .
</para>
</listitem>
<listitem >
<para >
Similarly, each Arabic character has four different variants
corresponding to the different positions in might appear in
within a sequence. Inside a font, there will be separate
glyphs for the initial, medial, final, and isolated forms of
each letter, each at a different glyph ID.
</para>
<para >
Unicode only assigns one codepoint per character, so a
Unicode string will not tell you which glyph variant to use
for each character. To decide, you need to analyze the whole
string and determine the appropriate glyph for each character
based on its position. In other words, <emphasis > text
shaping chooses the correct form of the letter by its
position and returns the correct glyph from the font</emphasis> .
</para>
</listitem>
<listitem >
<para >
Other languages involve marks and accents that need to be
rendered in specific positions relative a base character. For
instance, the Moldovan language includes the Cyrillic letter
" zhe" (ж) with a breve accent, like so: " ӂ" .
</para>
<para >
Some fonts will provide this character as a single
zhe-with-breve glyph, but other fonts will not and, instead,
will expect the rendering engine to form the character by
superimposing the separate " ж" and " ˘"
glyphs.
</para>
<para >
But exactly where you should draw the breve depends on the
height and width of the preceding zhe glyph. To find the
right position, you need to consult the table inside
the font (the <literal > GPOS</literal> table) that contains
positioning information.
In other words, <emphasis > text shaping tells you whether you have a
precomposed glyph within your font or if you need to compose a
glyph yourself out of combining marks— and, if so, where to
position those marks.</emphasis>
</para>
</listitem>
</itemizedlist>
<para >
If tasks like these are something that you need to do, then you need a text
shaping engine. You could use Uniscribe if you are writing
Windows software; you could use CoreText on macOS; or you could
use HarfBuzz.
</para>
<para >
In the rest of this manual, we are going to assume that you are the
implementor of a text-layout engine.
</para>
</section>
<section id= "what-harfbuzz-doesnt-do" >
<title > What HarfBuzz doesn't do</title>
<para >
HarfBuzz will take a Unicode string, shape it, and give you the
information required to lay it out correctly on a single
horizontal (or vertical) line using the font provided. That is the
extent of HarfBuzz's responsibility.
</para>
<para >
It is important to note that if you are implementing a
text-layout engine you may have other responsibilities that
HarfBuzz will <emphasis > not</emphasis> help you with. For example:
</para>
<itemizedlist >
<listitem >
<para >
HarfBuzz won't help you with bidirectionality. If you want to
lay out text that includes a mix of Hebrew and English, you
will need to ensure that each buffer provided to HarfBuzz has its
characters in the correct layout order. This will be different
from the logical order in which the Unicode text is stored. In
other words, the user will hit the keys in the following
sequence:
</para>
<programlisting >
A B C [space] ג ב א [space] D E F
</programlisting>
<para >
but will expect to see in the output:
</para>
<programlisting >
ABC אבג DEF
</programlisting>
<para >
This reordering is called <emphasis > bidi processing</emphasis>
(" bidi" is short for bidirectional), and there's an
algorithm as an annex to the Unicode Standard which tells you how
to reorder a string from logical order into presentation order.
Before sending your string to HarfBuzz, you may need to apply the
bidi algorithm to it. Libraries such as ICU and fribidi can do
this for you.
</para>
</listitem>
<listitem >
<para >
HarfBuzz won't help you with text that contains different font
properties. For instance, if you have the string " a
<emphasis > huge</emphasis> breakfast" , and you expect
" huge" to be italic, then you will need to send three
strings to HarfBuzz: <literal > a</literal> , in your Roman font;
<literal > huge</literal> using your italic font; and
<literal > breakfast</literal> using your Roman font again.
</para>
<para >
Similarly, if you change the font, font size, script,
language, or direction within your string, then you will
need to shape each run independently and output them
independently. HarfBuzz expects to shape a run of characters
that all share the same properties.
</para>
</listitem>
<listitem >
<para >
HarfBuzz won't help you with line breaking, hyphenation, or
justification. As mentioned above, HarfBuzz lays out the string
along a <emphasis > single line</emphasis> of, notionally,
infinite length. If you want to find out where the potential
word, sentence and line break points are in your text, you
could use the ICU library's break iterator functions.
</para>
<para >
HarfBuzz can tell you how wide a shaped piece of text is, which is
useful input to a justification algorithm, but it knows nothing
about paragraphs, lines or line lengths. Nor will it adjust the
space between words to fit them proportionally into a line. If you
want to layout text in paragraphs, you will probably want to send
each word of your text to HarfBuzz to determine its shaped width
after glyph substitutions, then work out how many words will fit
on a line, and then finally output each word of the line separated
by a space of the correct size to fully justify the paragraph.
</para>
</listitem>
</itemizedlist>
<para >
As a layout-engine implementor, HarfBuzz will help you with the
interface between your text and your font, and that's something
that you'll need— what you then do with the glyphs that your font
returns is up to you.
</para>
</section>
<section id= "why-is-it-called-harfbuzz" >
<title > Why is it called HarfBuzz?</title>
<para >
HarfBuzz began its life as text-shaping code within the FreeType
project (and you will see references to the FreeType authors
within the source code copyright declarations), but was then
extracted out to its own project. This project is maintained by
Behdad Esfahbod, and named HarfBuzz. Originally, it was a shaping
engine for OpenType fonts— " HarfBuzz" is the Persian
for " open type" .
</para>
</section>
</chapter>
